Pitches are hydrocarbon liquids that are solid at ambient temperature, but that melt upon heating. Upon further heating, volatile gases are evolved from the pitch, resulting in a solid residue that mainly comprises carbon. This solid residue is referred to as coke. In general, pitches may comprise a wide range of chemicals with a distribution of molecular weights, rather than a single component.
Conventional pitches may be obtained as byproducts from petroleum refining or metallurgical grade coke production ovens. In the latter case, volatile components evolved during the coking process are collected in the form of a tar. This tar may then be further refined by use of distillation to remove low boiling point materials, resulting in a pitch residue. Both petroleum pitches and coal tar pitches can be used for a variety of applications including use as binder pitch, anode pitch (i.e., an anode coke precursor pitch), graphite pitch, impregnation pitch, hard pitch, soft pitch, and others.
Raw coal generally is not considered to be a pitch because it devolatilizes before it can soften or melt. However, a synthetic pitch, also referred to as a “Synpitch,” can be created by extracting coal in a hydrocarbon solvent at an elevated temperature such that a molten slurry is formed. Typically, undissolved solids are removed by filtration or centrifugation and the solvent is removed via distillation. The softening point, coke yield, and aromatic content of the resultant pitch may be modified by distillation and/or air blowing.
A related process is direct liquefaction, in which coal is converted to an oil similar to crude petroleum (see William F. Taylor and Homer J. Hall, Future Synthetic Fuels: A Scientific and Technical Applications Forecast—1975, US Army Contract DAA05-73-C-0559, September 1975. See also Increased Automobile Fuel Efficiency and Synthetic Fuels: Alternatives for Reducing Oil Imports, NTIS order #PB83-126094, 1982). Direct liquefaction involves the use of hydrogen to chemically bond with molecules contained in coal, resulting in a more fluid material. One method by which hydrogen is transferred to coal is through the use of an intermediate “donor solvent” that contains excess hydrogen that is available to be transferred to hydrogen-poor molecules in coal. An example of a hydrogen donor solvent is tetrahydronaphthalene (tetralin), which contains four excess hydrogen atoms in each molecule. After transferring its excess hydrogen atoms to molecules contained in the coal, the tetralin is converted to naphthalene. Naphthalene can be re-converted to tetralin by exposing it to high temperature and high pressure in the presence of a metal catalyst. Examples of high pressure and high temperatures are 2500 psi and 425° C., respectively (reference: Increased Automobile Fuel Efficiency and Synthetic Fuels: Alternatives for Reducing Oil Imports, NTIS order #PB83-126094, 1982, p. 164. See also Baughman, Gary L., Synthetic Fuels Data Handbook. Second Edition, Denver, Colo.: Cameron Engineers, Inc. 1978).
The absorption of hydrogen by coal results in a more fluid material, especially at high temperature. By converting coal to a liquid in this manner, it is possible to remove insoluble solids, such as, but not limited to, fixed carbon and mineral matter, by either filtration or centrifugation, resulting in a nearly pure hydrocarbon liquid.
The process of dissolving coal in a solvent and reconstituting it without insoluble solids present is generally known as solvent extraction. It should be noted, however, that this is not a simple solution process. Rather, because of the large number of chemicals present in coal, there are a variety of chemical processes that can result in a phase change from solid to liquid. Specifically, it is thought that large molecular weight molecules contained in coal can be broken down into smaller molecular weight molecules by hydrogen-rich donor solvents. True chemical reactions are also thought to be likely to occur, as described above, such that hydrogen is physically transferred to certain molecules contained in coal. Thus, the solid-to-liquid phase change of coal in the presence of a liquid donor solvent likely consists not only of dissolution but also chemical digestion. For that reason it is more accurate to say that coal is “extracted” by the liquid, rather than simply “dissolved,” although outwardly the process resembles dissolution. Thus, as used herein, the term “extraction” (or “solvent extraction”) means the removal of material from coal (or other solids-containing mixture) by means of one or more solvents, wherein the removed (i.e., “extracted”) material simply dissolves in the solvent(s) and/or reacts to form a compound that is soluble in the solvent(s).
Alternative solvent extraction techniques for producing synthetic pitch from coal have been described previously. For example, Zondlo et al (U.S. Pat. No. 5,955,375, which is incorporated herein by reference) describe a solvent extraction process for coal using an NMP-type solvent. These solvents are described, for example, in U.S. Pat. No. 4,272,356 (hereinafter, “Stiller et al.”), which is also incorporated herein by way of reference. In the process described by Zondlo et al., the solvent is separated from the final product via evaporation or some other process. In typical solvent extraction processes for producing synthetic pitch from coal, the final product contains about 2% or less (by weight) of components derived from the solvent. In other words, at least 98% of the solvent is recovered and typically is recycled back to the extraction process. Because of the expense of the solvents typically used for producing synthetic pitch from coal, the process generally is not economical if more than a few percent of the solvent is left in the final product. In addition, the desired properties of the synthetic pitch may be adversely affected if too much solvent remains in the final product.
Blending is a well-known method to modify pitch properties. For example, Stansberry et al. describe how synthetic pitch may be blended with conventional coal tar pitch and other hydrocarbons in order to create modified pitches having certain desirable properties. Stansberry, P. G., J. W. Zondlo, and R. H. Wombles; Development of binder pitches from coal extract and coal-tar pitch blends; Light Metals, 581-585 (2001). However, the use of blended hydrocarbons as a solvent for extracting coal has not been previously described.